This invention relates to castable polyurethane and/or polyurethane/urea elastomer compositions with improved processing characteristics, including longer pour life, reduced tendency to crack, as well as better health and safety aspects since they are free of toluene diisocyanate. Isocyanate-endcapped prepolymers are employed in the castable elastomers of the invention. Effective processes for the production of such prepolymers and elastomers are disclosed. These prepolymers can be substituted for TDI-prepolymers and for aliphatic isocyanate based prepolymers with similar cure characteristics. The prepolymers of the invention, however, have improved health and safety aspects.
Aromatic polyisocyanates are well known and are widely used in the preparation of polyurethane and polyurethane/urea elastomers. These aromatic diisocyanates generally include compositions such as 2,4-toluene diisocyanate (2,4-TDI), 2,6-toluene diisocyanate (2,6-TDI), 4,4′-methylene-bis-(phenylisocyanate) and 2,4′-methylene-bis-(phenylisocyanate) (4,4′-MDI and 2,4′-MDI) and the like. In the preparation of polyurethane and polyurethane/urea elastomers, the aromatic diisocyanates are reacted with a long chain (high molecular weight) polyol to produce a prepolymer containing free isocyanate groups. This prepolymer may then be chain extended with a short chain (low molecular weight) polyol or aromatic diamine to form a polyurethane or polyurethane/urea elastomer (which is known generically as polyurethane or urethane). A liquid mixture of prepolymer and curative polymerizes, increasing steadily in viscosity until finally a solid elastomer is formed. Among the chain extenders or cross-linking agents (curatives) used, primary and secondary polyalcohols, aromatic diamines, and in particular, 4,4′-methylene-bis(2-chloroaniline), i.e. MBOCA, are most common. The use of MBOCA allows the manufacture of urethane elastomers with good mechanical properties and acceptable processing times.
Although MBOCA is the most widely used chain-extender in the production of castable polyurethanes, it suffers from the disadvantage of decomposition at high temperatures, as well as being quite toxic and Ames positive. These negative features of MBOCA have prompted those in the polyurethane art to investigate alternate materials as chain-extenders. Examples of other amines that have been used include 3,3′,5,5′-tetraisopropyl-4,4′-diaminodiphenylmethane and 3,5-dimethyl-3′,5′-diisopropyl-4,4′-diaminophenylmethane, 3,5-diethyl-2,4-toluenediamine and/or 3,5-diethyl-2,6-toluenediamine (i.e. DETDA), 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline), 3,5-dimethylthio-2,4-toluenediamine, 3,5-dimethylthio-2,6-toluenediamine, and 3,5-diamino-4-chlorobenzoic acid-isobutylester. While these amines do function as cross-linking agents, the resultant pot life of the polymer mixture is so short that a reasonable processing time for cast elastomers is not possible.
Another curing agent used in the manufacture of polyurethanes is methylene dianiline (MDA). Methylene dianiline is well-known to those skilled in the art as a good curative if there is only aliphatic diisocyanate present. It results in a much shorter pot life than MBOCA. This short pot life is exacerbated by the presence of toluene diisocyanate (TDI). There are also toxicity issues related to the use of MDA.
Another chain-extending agent for polyurethanes is 4,4′-methylene-bis(3-chloro-2,6-diethylaniline) (MCDEA, commercially available as Lonzacure from the Lonza Corporation). This curative material is reportedly lower in toxicity but it reacts with isocyanates much faster than MBOCA does. (See Th. Voelker et al, Journal of Elastomers and Plastics, 20, 1988 and ibid, 30th Annual Polyurethane Technical/Marketing Conference, October, 1986.) Although this curative does react with isocyanate-terminated prepolymers (including TDI-based prepolymers or 2,4′-MDI-based prepolymers) to give elastomers with desirable properties, they have a tendency to crack when undergoing polymerization.
The amount and presence of free, unreacted TDI monomer has other deleterious effects on the processing and manufacture of urethanes. A major problem with mono-nuclear aromatic diisocyanates, such as toluenediisocyanate, is that they are toxic and because of their low molecular weight, they tend to be quite volatile. Therefore, 2,4′-MDI-based prepolymers have much better health and safety aspects. Pure 4,4′-MDI-based prepolymers cured with amines, however, are much to fast.
U.S. Pat. No. 5,077,371 discloses a prepolymer that is low in free TDI. U.S. Pat. No. 4,182,825 also describes polyether based prepolymers made from hydroxy terminated polyethers capped with toluene diisocyanate, in which the amount of unreacted TDI is substantially reduced. These prepolymers can be further reacted with conventional organic diamines or polyol curatives to form polyurethanes. When combining the teachings of this patent with the use of MCDEA as a chain extender, the resulting solid elastomer goes through a gel stage having a low strength which can allow cracking of the polymerization mass to occur. Conventional TDI prepolymers with higher levels of free TDI also yield the same unsatisfactory gel state.
Surprisingly, it has been found that certain prepolymers prepared with both 2,4′-MDI and an aliphatic diisocyanate can be used with chain extenders such as 3,5-diamino-4-chlorobenzoacid isobutylester, to give elastomers with much longer casting time, thus providing more time and/or larger articles and/or a reduced propensity to crack. This phenomena was only known for TDI-based prepolymers prepared with both TDI and an aliphatic diisocyanate (see U.S. Pat. No. 6,046,297). The prepolymers of the present invention also provide extended pour life, and compared to TDI-based prepolymers known in this field, much better health and safety aspects since they are free of toxic TDI. MDI is known to have a much lower vapor pressure than TDI, and thus, is easier and safer to work with. An example of suitable aliphatic diisocyanate for the present invention would be a mixture of the three geometric isomers of 1,1′-methylene-bis-(4-isocyanato-cyclohexane), which are abbreviated collectively as “H12MDI.” One such mixture of isomers is available commercially and commonly referred to as dicyclohexylmethane-4,4′-diisocyanate. These results are surprising.